Powder flow measurement

ABSTRACT

The flow of a particulate material when conveyed hydrodynamically by means of a flowing fluid has a noise content consisting of random variations in the concentration of the particulate material. This noise content is sensed at two points separated by a known distance along the flow path of the material, and the sensed disturbances are cross-correlated to establish a transit time for a given disturbance between the two points, thus enabling the flow rate of the particulate material to be determined.

United States Patent Maurice Sidney Beck Cunningham et a1 [72] Inventors2,491,445 12/1949 73/194 10 Hazelhurst Road, Bradford 9, 3,184,9675/1965 Rogers .i 73/194 Yorkshire, England; 3,278,747 10/1966 Ohmart73/194 UX Andzrej Plaskowski, ul. Anielewicza 33 in 3,425,274 2/1969Clement et a1 73/194 Warsaw, Poland OTHER REFERENCES APPW 756969 E kh dt0 P t dA N 0 1 122 292 Jan. 1962 1221 Filed Sept. 3, I968 e [45]Patented July 27,1971 [73] Assignee National Research DevelopmentCorpora- 'm y Examinercharle5 Rueh] tion. London, EnglandAttorney-Cushman, Darby and Cushman [32] Priority Sept. 6, 1967 [33]Great Britain [31] 40688/67 [54] POWDER FLOW MEASUREMENT 11 Claims, 8Drawing Figs.

[52] U.S.C| 73/194 F, BS The flow of a particulate material when 73/28veyed hydrodynamically by means of a flowing fluid has a 511 11 c1 0011100 noise comm consisting of random variations in the Comm [50] F'eldSearch 73/ 194; tration of the particulate material. This noise contentis sensed 324/716] at two points separated by a known distance along theflow [56] Reeren Cited path of the material, and the sensed disturbancesare crossces correlated to establish a transit time for a givendisturbance UNITED STATES PATENTS between the two points, thus enablingthe flow rate of the par- 2,315,805 4/1943 Mayo et a1 73/194 ticulatematerial to be determined.

2) l l l 3 5 PATENTEB JULET nan SHEET 1 OF 3 i Q. v i 5L? 3?? N U IPATENTED JULZ'! l9?! SHEET 2 0F 3 078 di wwons V V U U 0U U U XSECONKDSF /G.3a.

T TRANSIT TIME .OF POWDER BETWEEN MB) ELECTRODES 3 AND 4 o F/G.3b.

* B SECONDS 4 (75) VOLTS Q POWDER FLOW MEASUREMENT The invention relatesto the measurement of the flow of particulate materials conveyedhydrodynamically by means of a flowing fluid, and is concerned moreparticularly, but not exclusively, with the measurement of the flow ofpowdered solids in pneumatic conveyors.

Pneumatic conveyors are simple, reliable and efficient means fortransporting powdered solid materials, and are used in many industrialprocesses.

The automatic control of such processes frequently requires thatcontinuous measurements of the flow rates of materials used in theprocess should be made.

This has presented difficulties when pneumatic conveyors are used fortransporting solid materials as the majority of solid material flowmeasuring devices will only operate when the solid material is not in afluidized state, and therefore require the removal of material from theconveyor, thus destroying some of the essential simplicity andreliability of such conveyors. Devices depending upon pressure-drops,analogous to venturi meters and pilot tubes, have been proposed but theyrequire that an obstruction should be placed in the conveyor to causethe pressure-drop, and there is therefore a risk of causing a blockageof the conveyor. Another disadvantage of such devices is that highaccuracy has not been attained due to the fact that the measuredpressure-drop is due to both the solid and the airflow.

A nucleonic method of measuring powder flow has been proposed whichinvolves no actual obstruction in the conveyor, but the method onlyworks satisfactorily with a gravityflow system and requires the materialto pass through a sharp double bend at the point of measurement; therisk of blockage of the conveyor therefore remains.

It has been observed that the flow of solid material in a pneumaticconveyor has a "noise" content which takes the form of random smallconcentrations and rarefactions that are superimposed upon the generalflow of material and are propagated along the conveyor with the samevelocity as the general flow.

It is to be understood that similar considerations will apply iffluids'other than air are used as the conveyor medium, and theparticulate material may be a liquid in droplet form.

According to the present invention there is provided a method of andapparatus for measuring the flow of a particulate material conveyedhydrodynamically by means of a flowing fluid, the method comprising thesteps of sensing the passage of random disturbances in the flow of thematerial past points separated by a known distance, andcross-correlating the sensed disturbances to establish a transit timefor the passage of the disturbance over the known distance; theapparatus according to the invention comprises at least two sensingelements separated by a known distance along a path for flow of thematerial, the sensing elements being adapted to detect small changes inthe quantity of material flowing past them, and means for deriving thetransit time between the sensing elements of random disturbances in theflow of the material by cross-correlation of signals generated by thesensing elements.

Preferably the particulate material is a powdered solid flowing in apneumatic conveyor and the sensing elements are capacitors built intothe wall of the conveyor, the disturbance being detected by changes inthe capacitances of the said capacitors.

The mass flow rate in a pneumatic conveyor can be determined if twoparameters are measured, these are:

l. the velocity of flow of the powdered material; 11(2) 2. The solidsloading in a unit length of the conveyor; w(!) If these quantities varywith time, then the total mass flow in T secs. is equal to:

Using the invention, these two parameters can be measured separately orsimultaneously, and, by way of example, an embodiment of the inventionwill be described in which the above parameters are measured separatelyand another embodiment will be described in which the two parameters aremeasured simultaneously.

In the accompanying drawings:

FIG. II shows diagrammatically an embodiment of the invention in whichthe velocity of solids flow and the known solids loading are measuredindependently;

FIG. 2 shows a typical output curve from a capacitance-toelectrictransducer used in the embodiment of FIG. 1;

FIG. 3a shows a plot of an autocorrelation function of the output fromthe transducers used in the embodiment of FIG.

FIG. 3b shows the impulse response of the flowpath;

FIG. 30 shows the cross-correlation function of the transducer outputs;and FIGS. 4a, db and dc are diagrammatic representations of amathematical analysis of the invention.

Referring to FIG. I, a pneumatic conveyor I has mounted in a wall 2 twoelectrodes 3 and ti that are separated by a known distance I. Each ofthe electrodes 3 and 4 consists of a portion of the wall 2 that isseparated from the remainder of the wall 2 by an insulating joint 5. Theelectrodes 3 and 4l each form capacitances with the remainder of thewall 2, the dielectric of which is the air/solid mixture flowing throughthe conveyor 1. Thus variations in the air/solid mixture will inducechanges in the capacitance of these capacitors. These changes incapacitance are sensed by respective capacitancc-to-electric transducers6, to which the electrodes 3 and 4 are connected. The transducers 6produce, in a manner to be described more fully later, signals that arerepresentative of small disturbances in the flow of solids past theelectrodes 3 and 4. These signals are processed in a computer 7, in amanner to be described more fully later, to give the transit time of anygiven disturbance in the flow from one electrode to the other. From thistransit time, as the separation l of the electrodes 3 and 4 is known,the velocity of solids flow is derived. A source 8 of 7- radiationpasses a beam of 'y-radiation through the conveyor I to a detector 9,and the solids loading of the conveyor 1 is derived in a known mannerfrom the amount of -y-radiation absorbed during its passage through theconveyor 1.

The transducers 6 are so designed that they are self-compensating forchanges in the standing capacitance of their respective electrodes, andare insensitive to low frequency changes in capacitance of theelectrodes. As the result of a large number of small capacitance changescaused by individual particles cross the field of the electrode 3,capacitance noise x(t) will be generated and FIG. 2 shows a typicaloutput signal from the transducer connected to electrode 3; that fromthe transducer connected to electrode 4 will be similar. This noise isGaussian noise which can be approximated by band-limited white noise,having a power spectrum @A f). By suitable design the transducers can bemade to have a cut-off frequency lower than the cut-off frequency of thepower spectrumtbflf).

The frequency spectrum of the output signal from the transducer coupledto the electrode 3 is:

mm lm where K 6 (if) is the frequency response of the transducer.

The transducer output signal is band-pass random noise, itsautocorrelation taking the form I mm. shown in FIG. 3a.

The velocity measuring section of the pneumatic conveyor I between theelectrodes 3 and d (FIG. I) can be represented by the model in FIG. 4a.The noise at the electrode 3 is x(t): a fraction K xfl) is delayed by apure time delay 1', giving a contribution y (t) at electrode 4i, whereris the transit time of the power betweenthe electrodes. This delayednoise y(t) cannot be measured directly, because there will be a certainamount of random decomposition of the noise pattern as the powdertravels between the electrodes. Following the principle of linearsuperposition the decomposition can be represented by a single spuriousnoise z(t), which is added to the delayed noise y(!) to give a measuredcapacitance noise y(t) at the electrode 4.

The block diagram model (FIG. 4a) can be regrouped as shown in FIG. 4b.A further simplification can then be made because the transducers ineach measurement channel are identical. Hence any phase delay in thetransducers will cancel out and does not affect the relative phase ofthe transducer output signals denoted by m(t) and n(!), respectively.This leads to the model in FIG. 40.

The velocity of solids flow is derived from the transit time of the flowdisturbances, which are found by using correlation techniques. This isnecessary because the traces of the outputs from the two electrodes 3and 4 are not the same, due to the random effects that occur during thetime of transit from electrode 3 to the electrode 4, and the passage ofany given disturbance between the electrodes 3 and 4 is not immediatelyapparent.

The transit time of a disturbance between the electrodes 3 and 4 isderived as follows.

It can be shown that if two functions 11(1) and [2(1) are related by atransfer function of the form:

b(r)=a(l-1,) 3 where a(t) is a nonperiodic function which can beconsidered to be statistically stationary over a period of time 5, thenthe cross-correlation function between them which is given by:

will have a maximum value when y= r,

where r, is the time delay of the system, that is when the correlationtime delay is equal to the pure time delay of the system. The outputsignal from the transducer connected to electrode3 is m(t) and that fromthe transducer connected to the where B represents a second timevariable, h(B) is the impulse response of the model in FIG. 4c and E('y)is the expected error in the estimate of 1 ,,,,,('y) caused by thefiltered spurious noise z(t). Also, the cross-correlation function ofthe signals m(t) and n(t) is given by:

and the autocorrelation of the signal m(l) is given by (r f (i) 1 i m 7(8) The impulse response [1(5) of the model (FIG. 40) has the values:

MBF-K when B=r h(fi)=0 when [#7 9 From (9) the integral in equation (6)is single valued when B= and zero elsewhere therefore mn('Y) 3mn('y (7)U The relationship between l ,,,,,(-y) and (y) will thus take the formshown in FIG. 3c, neglecting the error E('y). The time delay of themaximum value of the cross-correlation function I ,,,,,(-y) occurs wheny=r which is the transit time of the powder between the electrodes 3 and4. The velocity of solids flow is then given by v=l/r and the mass flowrate will be given by:

l L t Where k,, is the present value of the sampling evolutionaryvariable, ,=8L, where l in an integer; y=i8 where i is an integer; and Adenotes the deviation from the average value. The use of the deviationdoes not alter the shape of the correlation, it causes the correlationfunction to have a zero mean value and removes the possibility ofnumerical overflow in the computer.

The computer 7 can be an online digital computer having a high speedinput multiplexer to read the signals m(t) and n(t), as shown in FIG. l.A suitable sampling interval for rnultiplexer could be 0.5 ms. Thecomputer is programmed to calculate equation (l2) for different valuesof i, to locate the maximum value of the function 1 (i) and hence thecorresponding value ofi and to calculate the mass flow rate from theequation (I I) if a nucleonic method is employed to measure the loadingof powder. Alternatively, in the second embodiment equation 13) belowcan be used.

It is to be noted that in the embodiment described above no assumptionshave been made about the relationship between the output of thetransducers and the loading of the conveyor I and therefore saturationof the output signals from the trans ducers is acceptable. However, in asecond embodiment of the invention transducers are used, the outputs ofwhich are linearly related to the loading in the conveyor 1, or arerelated in some other determinate manner in order that the averageloading of the conveyor ll may be determined. This may be done bymodification to the transducers used in the first embodiment if thepermittivity of the conveyed material is either known or can bemeasured.

The permittivity can be measured by a capacitance electrode and suitabletransducer which is positioned either in a feed hopper (not shown) or ina hopper into which the powder is discharged (also not shown). In thiscase care must be taken to ensure that the output signals from thetransducers 6 do not saturate.

In such cases, where the permittivity of the powder is either sensiblyconstant, or it only varies as a result of moisture changes and it canbe measured, it can be shown that the velocity and loading can both bedetermined from the crosscorrelation function given by equation (12),giving:

i )l= 4 mn* V/ (H) where [M(t)] is the mass flow averaged over anintegration period if, corresponding to L in equation (12), K is acalibration constant for the installation, 6 is the permittivity of thepowder, 1 *,,,,,(i) is the maximum value of the cross-correlationfunction I ,,,,,(i), and i* is the corresponding value ofi.

What we claim:

1. A method of measuring the flow of a particulate material conveyedhydrodynamically by means of a flowing fluid, said method comprisingobtaining first and second signals by sensing the passage of naturallyoccuring random disturbances in the flow of the particulate materialrespectively past first and second points separated by a known distancealong a path for flow of the material, and

deriving the transit time of the particulate material over said knowndistance by ascertaining that value of the time delay between said firstand second signals for which the cross-correlation function of saidfirst and second signals has its maximum value.

are variations in the concentration of-thc particulate material in saidfluid.

3. A method according to claim 2, including the step of measuring themass loading of the particulate material per unit length of the path forflowof the material.

4. A method according to claim ll wherein the particulate material is apowdered solid and said pathfor flow of the particulate material isprovided by a pneumatic conveyor.

5. Apparatus for measuring the flow of a particulate material conveyedhydrodynamically by a flowing fluid along a give path, said apparatuscomprising a first sensing element operative to generate a first signalcorresponding to small random changes in the quantity of the particulatematerial flowing past a first point in said given path,

a second sensing element operative to generate a second signalcorresponding to small random changes in the quantity of the particulatematerial flowing past a second point in said given path,

said first and second points being separated by a known distance alongsaid path, and

means for ascertaining that value of the time delay between said firstand second signals for which the cross-correlation function of saidfirst and second signals has its maximum value.

6. Apparatus according to claim 5, wherein said path is pro vided by apneumatic conveyor.

7. Apparatus according to claim 5, wherein each of said sensing elementsincludes an electrode, disposed so that variations in the quantity ofsaid material flowing past the electrode causes corresponding changes inthe capacitance of the electrode.

8. Apparatus according to claim 7,whcrein said electrodes form part of awall of a pneumatic conveyor.

9. Apparatus according to claim 5, including means for measuring themass loading of material per unit length of said path.

ll. Apparatus according to claim '9, wherein said means for measuringthe mass loading per unit length of said path comprises a source ofradiation situated on one side of said path and detecting means situatedon the opposite side of said path.

11. Apparatus for measuring the flow of a particulate material conveyedhydrodynamically by a flowing fluid along a given path, said apparatuscomprising a first electrode disposed at a first point in said givenpath so that small random changes in the quantity of the particulatematerial flowing past said first point cause corresponding changcs inthe capacitance of the first electrode,

a second electrode disposed at a second point in said given path so thatsmall random changes in the quantity of the particulate material flowingpast said second point cause corresponding changes in the capacitance ofthe second electrode,

said first and second points being separated by a known distance alongsaid path,

a first transducer means for producing a first electrical signal relatedto the changes in capacitance of said first electrode,

a second transducer means for producing a second electrical signalrelated to the changes in capacitance of said second electrode, and

means for ascertaining that value of the time delay between first andsecond signals for which the cross-correlation function of said firstand second. signals has its maximum value.

1. A method of measuring the flow of a particulate material conveyedhydrodynamically by means of a flowing fluid, said method comprisingobtaining first and second signals by sensing the passage of naturallyoccuring random disturbances in the flow of the particulate materialrespectively past first and second points separated by a known distancealong a path for flow of the material, and deriving the transit time ofthe particulate material over said known distance by ascertaining thatvalue of the time delay between said first and second signals for whichthe crosscorrelation function of said first and second signals has itsmaximum value.
 2. A method according to claim 1, wherein thedisturbances are variations in the concentration of the particulatematerial in said fluid.
 3. A method according to claim 2, including thestep of measuring the mass loading of the particulate material per unitlength of the path for flow of the material.
 4. A method according toclaim 1 wherein the particulate material is a powdered solid and saidpath for flow of the particulate material is provided by a pneumaticconveyor.
 5. Apparatus for measuring the flow of a particulate materialconveyed hydrodynamically by a flowing fluid along a give path, saidapparatus comprising a first sensing element operative to generate afirst signal corresponding to small random changes in the quantity ofthe particulate material flowing past a first point in said given path,a second sensing element operative to generate a second signalcorresponding to small random changes in the quantity of the particulatematerial flowing past a second point in said given path, said first andsecond points being separated by a known distance along said path, andmeans for ascertaining that value of the time delay between said firstand second signals for which the cross-correlation function of saidfirst and second signals has its maximum value.
 6. Apparatus accordingto claim 5, wherein said path is provided by a pneumatic conveyor. 7.Apparatus according to claim 5, wherein each of said sensing elementsincludes an electrode, disposed so that variations in the quantity ofsaid material flowing past the electrode causes corresponding changes inthe capacitance of the electrode.
 8. Apparatus according to claim 7,wherein said electrodes form part of a wall of a pneumatic conveyor. 9.Apparatus according to claim 5, including means for measuring the massloading of material per unit length of said path.
 11. Apparatus formeasuring the flow of a particulate material conveyed hydrodynamicallyby a flowing fluid along a given path, said apparatus comprising a firstelectrode disposed at a first point in said given path so that smallrandom changes in the quantity of the particulate material flowing pastsaid first point cause corresponding changes in the capacitance of thefirst electrode, a second electrode disposed at a second point in saidgiven path so that small random changes in the quantity of theparticulate material flowing past said second pOint cause correspondingchanges in the capacitance of the second electrode, said first andsecond points being separated by a known distance along said path, afirst transducer means for producing a first electrical signal relatedto the changes in capacitance of said first electrode, a secondtransducer means for producing a second electrical signal related to thechanges in capacitance of said second electrode, and means forascertaining that value of the time delay between first and secondsignals for which the cross-correlation function of said first andsecond signals has its maximum value.
 11. Apparatus according to claim9, wherein said means for measuring the mass loading per unit length ofsaid path comprises a source of radiation situated on one side of saidpath and detecting means situated on the opposite side of said path.